Entropy, vortex interactions, and the phase diagram of heavy-ion irradiated Bi₂Sr₂CaCu₂O₈

Dynamic and thermodynamic magnetization experiments on heavy-ion irradiated single crystalline Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} are correlated in order to clarify the nature of the mixed state phase diagram. It is shown that whereas the entropy contribution to the free energy in the London regime plays a minor role in unirradiated crystals and irradiated crystals at fields close to or above the matching field $B_{\phi}$, it becomes very important at low fields in irradiated crystals with high $B_{\phi}$. The direct determination of the entropy contribution to the free energy from the reversible magnetization allows one to determine not only the correct values of the pinning energy, but also to extract quite detailed information on pancake vortex alignment. The characteristic field $H_{int} \sim {1/6} B_{\phi}$ at which intervortex repulsion begins to determine the vortex arrangement and the reversible magnetization is shown to coincide with a sharp increase in the irreversibility field $H_{irr}(T)$ and with the recoupling transition found in Josephson Plasma Resonance. Above $H_{int}$, the repulsive interaction between vortices cause both the vortex mobility to decrease and pancake alignement to increase. At higher fields $\gtrsim {1/3}B_{\phi}\gg B_{c1}$, free vortices outnumber those that are trapped on a columnar defect. This causes the decrease of c-axis resistivity and a second crossover of the irreversibility field, to a regime where it is determined by plastic creep.